The present invention relates generally to the manufacture of circuit boards and pertains, more specifically, to the placement of pads of solder paste in a prescribed pattern on a printed circuit board being prepared for the surface mounting of components onto the circuit board by subsequent soldering.
Current manufacturing techniques for making printed circuit boards call for the mounting of components to the circuit board by xe2x80x9csurface mountingxe2x80x9d. That is, rather than passing the leads of a component through holes in a circuit board and then soldering the leads at the holes, the circuit board is provided with pads of solder paste and the components are placed on the pads for soldering directly to the surface of the board.
In order to place the pads of paste in appropriate locations on the circuit board, just before affixing the components to the board, a stencil-like screen is juxtaposed with the circuit board and provides a pattern of openings registered with the locations where the pads are to be placed. The paste then is spread over the screen with a squeegee to lay down the pattern of pads. The stencil screen is in the form of a thin foil which is supported by a stretcher frame in an appropriately tensioned state. The frame fits into a machine which registers the tensioned screen with the circuit board and then applies the paste. Typically, the machine also stretches the stencil to achieve the desired tension of the stencil to ensure accuracy during printing.
However, the foregoing stencils pose serious safety and operational problems. For example, traditional foil stencils are potential safety hazards to those technicians that handle them due to the presence of sharp edges around the entire periphery of the foil stencil. Such sharp edges can easily cut a person handling the foil stencil and putting them at risk of exposure to infection. This is a particular concern due to the presence of solder-paste, which can be toxic, in the work environment. In light of the aforementioned safety hazards, transportation and storage of foil stencils are problematic because technicians are constantly transporting foil stencils to and from a storage location during the normal course of circuit board manufacture. During this normal handling, the foil stencil undergoes bending, for example, that threatens its integrity. As a result, foil stencils are prone to damage.
There have been many attempts in the prior art to address the foregoing problems with foil stencils relating to safety to the handler and damage during handling and storage. For example, it is common in the prior art to mount foil stencils to an aluminum frame with a polyester border to ensure proper tensioning of the foil stencil. While the encapsulation of the periphery of the foil stencil with polyester keeps the foil tight and covers the sharp edges from the handler, it is time consuming and expensive to encapsulate the foil stencil and requires special molding machinery. As a result, such polyester encapsulated foil stencils require an additional preparation step which typically requires outsourcing to a special subcontractor for such operation. Moreover, these encapsulated foil stencils are difficult to store and cannot be easily retrieved when stacked.
In view of the foregoing, there is a demand for a frame for a foil stencil assembly that protects that handler from the sharp edges to avoid injury. There is a further demand for a foil stencil assembly that can be easily stored and retrieved. There is a demand for a foil stencil assembly that can lay flat or hang from a vertical hook in a high density storage cabinet. There is a further demand for a foil stencil assembly that is inexpensive and easy to assemble without special tools or special equipment. There is also a demand for a foil stencil assembly that has stencil tension that does not decrease over time and is machine washable. In addition, there is a further demand for a foil stencil assembly with a large print area.
The present invention preserves the advantages of prior art stencil foil assemblies for the surface mount solder paste stencil printing industry. In addition, it provides new advantages not found in currently available assemblies and overcomes many disadvantages of such currently available assemblies.
The invention is generally directed to a stencil foil assembly includes a stencil foil, having a stencil pattern thereon, of a planar configuration. The assembly also includes a rigid frame having a generally L-shaped profile with an upstanding wall and a support flange emanating inwardly from the upstanding wall. The stencil foil resides on the support flange with the bottom surface of the periphery of the stencil foil being in communication with the top surface of said support flange. The outer edge of the stencil foil is positioned adjacent to the inner surface of said upstanding wall. The periphery of the stencil foil is secured to the rigid frame. The stencil foil assembly protects a user from injury by preventing contact with the sharp outer edges of the stencil foil. The assembly protects the stencil foil portion from damage, maintains it in a tensioned condition and greatly facilitates handling thereof.
The stencil foil may be secured to the support flange of the outer rigid frame in a number of different ways. Preferably, a number of studs are provided on the support flange which engage with mounting holes on the stencil foil itself. The mounting holes are, preferably, of a star washer configuration whereby they grippingly engage with the studs on the support flange. For assembly, the outer rigid frame is positioned with the top surface of the support flange facing upwards. The shape of the outer rigid frame and the stencil foil are substantially identical. The stencil foil is positioned over the support flange so that the studs of the support flange align with their corresponding mounting holes. The periphery of the stencil is pushed down onto the support flange to snap the stencil foil into place thereby securing the stencil foil to the outer rigid frame in preparation for handling and subsequent use.
It is therefore an object of the present to provide a stencil foil assembly that protects the user from injury from sharp edges during handling of the foil.
Another object of the present invention is to provide a stencil foil assembly that protects the stencil foil from damage during shipping and handling.
It is a further object of the present invention to provide a stencil foil assembly that is inexpensive and easy to manufacture and assemble.
It is a further object of the present invention to provide a stencil foil assembly that enables a larger printing area than prior art stencil foil assemblies.
It is another object of the present invention to provide a stencil foil assembly that has reduced storage requirements.
A further object of the present invention is to provide a stencil foil assembly that allows shorter manufacturing turn around times.
Yet a further object of the present invention is to provide a stencil foil assembly that eliminates the need to return used frame castings.
Another object of the present invention is to provide a stencil foil assembly that reduces setup time and the need for tools for such setup.
A further object of the present invention is to provide a stencil foil assembly where the stencil tension does not decrease with extensive use.
Another object of the present invention is to provide a stencil foil assembly that is machine washable.